Lunar gravity-maps

These print-resolution stills were created for the
cover of the February 8, 2013 issue of Science. They show the
free-air gravity map developed by the Gravity Recovery and Interior
Laboratory (GRAIL) mission.

If the Moon were a perfectly smooth sphere of uniform density, the
gravity map would be a single, featureless color, indicating that
the force of gravity at a given elevation was the same everywhere.
But like other rocky bodies in the solar system, including Earth,
the Moon has both a bumpy surface and a lumpy interior. Spacecraft
in orbit around the Moon experience slight variations in gravity
caused by both of these irregularities.

The free-air gravity map shows deviations from the mean, the
gravity that a cueball Moon would have. The deviations are measured
in milliGals, a unit of acceleration. On the map, dark purple is at
the low end of the range, at around -400 mGals, and red is at the
high end near +400 mGals. Yellow denotes the mean.

These views show a part of the Moon's surface that's never visible
from Earth.

They are useful for understanding the subsurface structure, physical processes and history of the moon. The moon and earth are related in many respects (e.g. where we are in the solar system), but the moon tends to preserve structure that is obliterated by fast weathering and plate tectonics. So the maps are also related to earth’s history.

If you’re a geologist gravity anomalies are incredibly useful for giving some indication of the internal structure of a body. The physical size, rate of change and magnitude of gravitational anomalies let you begin to map where material of greater or lesser density is located inside the body – is it a shallow magmatic intrusion, is it molten or partially molten, is it something deep down and so on…
There’s a pronounced asymmetry in the appearance of the Moon between front-side and far-side with far more large infilled impact basins on the side closest to Earth. These are associated with similar differences between the gravitational patterns of the two hemispheres.